rust: clarify the language unstable features in use

We track the details of which Rust features we use at our usual "live
list" [1] (and its sub-lists), but in light of a discussion in the LWN
article [2], it would help to clarify it in the source code.

In particular, we are very close to rely only on stable Rust language-wise
-- essentially only two language features remain (including the `kernel`
crate).

Thus add some details in both the feature list of the `kernel` crate as
well as the list of allowed features.

This does not over every single feature, and there are quite a few
non-language features that we use too. To have the full picture, please
refer to [1].

Link: https://github.com/Rust-for-Linux/linux/issues/2 [1]
Link: https://lwn.net/Articles/1015409/ [2]
Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250327211302.286313-1-ojeda@kernel.org
[ Improved comments with suggestions from the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/Makefile.build

@@ -222,6 +222,15 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
+# The features in this list are the ones allowed for non-`rust/` code.
+#
+#   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
+#   - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`.
+#   - Stable since Rust 1.87.0: `feature(asm_goto)`.
+#   - Expected to become stable: `feature(arbitrary_self_types)`.
+#
+# Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
+# the unstable features in use.
 rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the